<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div onclick="console.log('first div')">
      <div onclick="console.log('second div')">
        <button onclick="console.log('button')">Click!</button>
      </div>
    </div>
  </body>
  <script>
    console.log(
      [
        [0, 1],
        [2, 3],
      ].reduce(
        (acc, cur) => {
          return acc.concat(cur);
        },
        [1, 2]
      )
    );
  </script>
</html>
